A traditional railway track structure includes rails, fasteners, sleepers, ballast, and a subgrade which underlies the ballast.
The track ballast is customarily crushed stone whose purpose is to support the sleepers and allow some adjustment of their position, while allowing for water drainage.
Ballast should be strong, hard-wearing, stable, drainable, easy to clean, workable, resistant to deformation, easily available, and reasonably cheap to purchase.
Good quality track ballast is made of crushed natural rock with particles between 28 mm and 50 mm in diameter, a high proportion of particles finer than this will reduce its drainage properties, and a high proportion of larger particles results in the load on the sleepers being distributed improperly. Angular stones are preferable to naturally rounded ones, as angular stones interlock with each other, inhibiting track movement.
Typically, ballast is laid to a thickness of at least 300-400 mm. An insufficient depth of ballast can result in the underlying soil being overloaded and, in the worst cases, the track can sink.
Over time, the ballast of a railway track is crushed into smaller pieces by the weight of trains passing over it. This crushing can make the ballast unstable and when combined with the effects of dust and rain on the ballast can effectively cause the ballast to form a dam wall which is difficult or impossible for water to drain through. As a consequence, water may build-up on one side of an affected railway track, particularly if the track is located on a flood plain which is subjected to flooding.
If water does build-up beside a railway track to a sufficient extent it can push on the ballast of the track with sufficient force to displace the ballast. This in turn can displace the sleepers which are supported by the ballast as well as the rails which are supported by the sleepers so that the track becomes kinked.
Although the track may still be useable if the ballast is crushed or if the track is not too badly kinked, it is usually necessary for trains to reduce their speed so that they can safely negotiate the affected portion of track. For example, a train with empty wagons may have to reduce its speed to as little as 30-35 km/h in order to negotiate the affected portion of track.
To prevent this problem from occurring, ballast should be periodically cleaned and, if necessary, replaced, to ensure that it is able to adequately drain water. However, due to the expense of performing such maintenance, it is often not performed as often as it should, or even at all, in some cases.
The laying of a modern railway track typically involves leveling, grading, and compacting to a specific resistance the ground/subgrade on which the track is to be laid. Geo cloth is then placed on top of the ground, and then a layer of ballast is then placed on top of the cloth. Although water can drain through the ballast, it usually meets resistance when it encounters the geo cloth.
It sometimes happens that part of the foundation of a railway track will be washed away by water such that a channel in which water collects is created beneath the ballast of the track. As a consequence, the ballast above the channel is not properly supported by the foundation. When the wheels of a train pass over this region of the track, they can repeatedly press the part of the track above the channel into the channel and cause water in the channel which contains particles of dirt and other materials to be pumped out on to the track, including the track ballast. This dirty water can contaminate the track which can foul the ballast and prevent it from draining water properly which can lead to water displacing the ballast as described earlier.
Sometimes track ballast will not just be displaced by water it will be completely washed away. When this happens it can result in the affected track being shut down, and can also result in equipment and goods being lost or damaged.
Some railway bridges include a corrugated or channeled steel plate which supports the ballast of a railway track. At the bottom of the corrugations/channels there are drainage holes for allowing water which drains through the ballast into the channels to drain out of the channels so that it does not cause the plate to rust away. Over time, as the ballast is crushed by the weight of trains which pass over it and smaller particles of the ballast fall into the channels along with other material, the drainage holes in the plate become clogged so that water is unable to drain from them properly if at all. Consequently, the plate begins to rust away.
The sleepers of a railway track that are supported by ballast and that are located on a corner of the track are particularly prone to moving/drifting apart as trains pass over them. This is a consequence of the various forces exerted on the track by the trains, as well as the vibrations that they are subjected to.
Railway tracks in sandy environments are prone to being covered by sand. For example, in Saudi Arabia as well as many other Middle Eastern countries, railway tracks are prone to being buried by desert sand which drifts on to the tracks.
Typically, a bulldozer is used to clear the sand from the tracks. However, this can often cause damage to the track, including the sleepers. Moreover, the track can become unaligned, and it is usually not possible to realign the track until all of the sand covering it has been removed.
If the track is of the sleeper rail type, once the track is laid it is virtually impossible to modify or adjust the track in an attempt to try and prevent it from being buried under sand.
Also, in hot environments, the use of ballast to support the sleepers of a railway track can increase the heat of the rails of the track to such an extent that they buckle. In particular, ballast that is used in railway track structures tends to trap heat which can heat the rails that are supported by the sleepers that are in turn supported by the ballast. If the rails are heated to a sufficient extent, they can distort or buckle.
Skilled workers are usually required to lay the ballast of a traditional railway track. This can increase the expense of laying such a track, particularly in times when such workers are hard to come by.
Moreover, the construction of a traditional railway track which includes ballast laid on a subgrade, a plurality of sleepers supported by the ballast, and rails supported by and secured relative to the sleepers can be time consuming and therefore expensive.
In addition, in an effort to prolong their use for as long as possible with as little maintenance as possible, railway tracks are usually laid on good quality land with stable ground, etc. This obviously prevents the land from being used for other more useful purposes such as farming.
Another disadvantage of railway tracks which utilise ballast is that when the track bed becomes uneven, it is necessary to pack ballast underneath sunken sleepers to level the track out against. This operation is usually done by a ballast tamping machine. Tamping the ballast will often increase the overall height of the ballast which can make the ballast less stable.
A further disadvantage of railway tracks which utilise ballast is that, at a railway junction having a switch enabling trains to be guided from one railway track to another, pieces of ballast may become lodged between points or switch rails of the switch and diverging outer rails or stock rails of the junction. Such lodgement may inhibit their movement between required switching positions and in some cases prevent required switching positions from being adopted. This can lead to delays and shut downs on the affected track as the lodged pieces of ballast are removed, and can also result in equipment and goods being lost or damaged and persons being injured or killed if the problem is not identified in sufficient time.
Movement of the points or switch rails of the switch may also be inhibited in cold weather conditions which may result in them freezing and becoming “sticky” or locked.
The heavy demand for maintenance is a significant disadvantage of railway tracks which utilise ballast to support sleepers. In particular, the heavy demand for surfacing/tamping and lining to restore the desired track geometry and smoothness of vehicle running is a significant disadvantage. Weakness of the subgrade and drainage deficiencies also leads to heavy maintenance costs. This can be overcome by using ballastless track. In its simplest form this consists of a continuous slab of concrete with the rails supported directly on its upper surface using a resilient pad which is attached by bolts thereto. Over time, without maintenance (which is often not conducted as regularly as it should because of the costs involved), the bolts may become loose or dislodged, resulting in movement of the pad. This movement can result in the rails becoming unsupported or unstable, which can lead to delays and shut downs on the affected track, and can also result in equipment and goods being lost or damaged and persons being injured or killed.
There are a number of proprietary systems, and variations include continuous in situ placing of a reinforced concrete slab, or alternatively the use of pre-cast pre-stressed concrete units laid on a base layer.
However, ballastless track is very expensive to construct, and in the case of existing railroads requires closure of the route for a somewhat long period. Its whole life cost can be lower because of the great reduction in maintenance requirement. Ballastless track is usually considered for new very high speed or very high loading routes, in short extensions that require additional strength (e.g. a rail station), or for localised replacement in the case of exceptional maintenance difficulties.
It is against this background that the present invention has been developed.